As LED (Light Emitting Diode) features environment-friendliness, high power-efficiency, lightweight, long service life, low operating voltage and high impact-resistance, it has gradually replaced the conventional fluorescent lamp and incandescent lamp and widely applied to head lights, direction lights, indicator lights, illuminators and displays. In the existing technology, LED is a solid-state element, which is primarily made of nitrides having a direct band gap and high breakdown voltage. The GaN (gallium nitride) LED is conventionally grown on the c-plane. However, the intrinsic electric field thereof deflects the structure of the QW (Quantum Well) energy band and decreases the probability of the overlap of the wave functions of electrons and holes. Thus the light emitting efficiency of LED is reduced. Therefore, the manufacturers desire to develop a semipolar GaN free of intrinsic electric field so as to increase the internal quantum efficiency of LED.
A Japan Publication No. 2008305977 disclosed a technology for growing semipolar GaN on a silicon substrate. In the prior art, ridged periodic mask patterns made of silicon dioxide are defined parallel to <−110> direction on a 7-degree off (001) silicon substrate.
Next, a KOH solution is used to etch the silicon substrate to form grooves having two inclined faces, wherein one of the two inclined faces is a (111) crystallographic plane intersecting with the primary plane of the silicon substrate, and the other is a (−1-11) crystallographic plane intersecting with the primary plane of the silicon substrate. Next, silicon dioxide covers on the silicon substrate except the (111) crystallographic plane, and a GaN film is grown on the (111) crystallographic plane with the (1-101) crystallographic plane of the GaN film on the (111) crystallographic plane of the silicon substrate. Thereby a semipolar GaN film is formed. However, there is lattice mismatch existing between the GaN film and the (111) crystallographic plane. Further, there is great difference of the thermal expansion coefficients thereof Thus about −50% tensile stress is generated that usually causes the semipolar GaN film to crack along the <110> direction.